There are many types of digital-to-analog controllers (DACs), which convert digital signals (i.e., a representation of a signal using discrete numbers) into continuous time signals (i.e., an analog signal). Currently, many high-resolution DACs implement a multistage dual string DAC topology using two sets of resistor strings to construct an output voltage from a digital signal, with the first string DAC implementing the most significant bits (MSBs) of the digital signal and the second string DAC implementing the least significant bits (LSBs) of the digital signal. The two voltages from the DAC strings are summed together to generate the analog signal. A string DAC is a type of static DAC including a plurality of resistors connected in series between a high reference voltage and a low reference voltage, where the various connecting nodes between the resistors constitute tap points that are selectively switched to an output node in response to the digital input, and the voltage of the tap point selectively switched to the output node is an inherently monotonic analog representation of the digital input.
Another type of multistage DAC is an interpolating DAC including a string DAC and a LSB segmented interpolation DAC. The interpolating DAC uses an interpolation circuit to interpolate between two voltages provided by the string DAC. The string DAC provides a portion of the analog signal corresponding to the MSB of a digital signal, while the interpolation DAC provides the portion of the analog signal corresponding to the LSB of the digital signal. Additionally, the interpolation DAC and the string DAC are inherently monotonic to ensure that the entire DAC is monotonic. The string DAC allows for larger coarse voltages for the MSB of a digital signal, while the interpolation DAC uses differential pairs of transistors to accurately interpolate between the two voltages from the string DAC. The interpolating DAC design increases the effective resolution and accuracy of a DAC while minimizing the number of components and energy consumption. As computing, sensing, motor control, process control and feedback control system require greater precision, the accuracy of a DAC becomes more important because sources of error in DACs limit the resolution accuracy. Currently, for 12-16 bit DACs, which implement LSB interpolating circuits, error is on the order of 1-3 bits, thereby limiting the precision of applications.